With recent advance in optical applications technology, shorter wavelengths of laser sources are increasing in demand.
This is because a shorter wavelength laser can improve the recording density and photosensitivity, and such a shorter wavelength laser is expected to be applied to optical devices such as optical discs, laser printers, and the like.
Therefore, there have been intensively conducted studies on second harmonic wave generating (SHG) devices which are capable of converting wavelengths of incident laser light to 1/2.
Heretofore, bulk single crystals of nonlinear optical crystals have been used as such second harmonic wave generating (SHG) devices, using high-output gas lasers as light sources. However, since there has been a strong demand for compact optical disc devices, laser printers, and the like, and gas lasers, which use optical modulation, require external modulators, semiconductor have been increasingly and predominantly used in place of gas lasers, for their capability of direct modulation and low costs. For this purpose, thin film waveguide SHG device has become in demand in order to obtain high conversion outputs with low light source outputs of several mW to several tens of mW.
Heretofore, as nonlinear optical materials for use in such thin film waveguide SHG devices, there have been known a lithium niobate bulk single crystal which is diffused with Ti or the like to form a waveguide layer having a different refractive index, and a lithium tantalate substrate on which a lithium niobate single crystal thin film is formed as a waveguide by radio-frequency sputtering or the like, these methods have been difficult to obtain lithium niobate single crystal thin films of good crystallinity, and it has thus been impossible to obtain high conversion efficiency.
Liquid phase epitaxy method is considered to be advantageous as a method to fabricate single crystal thin films with good crystallinity.
Liquid phase epitaxial methods to obtain lithium niobate single crystal thin films are described, for example, in
1) Japanese Patent Publication 51-9720/1976, and Applied Physics Letters, Vol. 26, No. 1, January 1975 p8-10, in which a lithium niobate thin film for optical waveguide is formed on a lithium tantalate substrate by a liquid phase epitaxial method, using Li.sub.2 O or V.sub.2 O.sub.5 as a flux.
2) Japanese Patent Publication 56-47160/1981, in which a lithium niobate/lithium tantalate solid solution thin film containing Mg is formed by an epitaxial technique on a substrate using Li.sub.2 O or V.sub.2 O.sub.5 as a flux.
However, with liquid phase epitaxial methods which have heretofore been known, it is impossible to obtain a lithium niobate single crystal thin film with good crystallinity on a lithium tantalate substrate, and, it is particularly difficult to obtain a lithium niobate single crystal thin film having a thickness required for fabrication of SHG devices and having low propagation losses. There have been known no thin film waveguide SHG devices which have been practically applied.
The film thickness required to fabricate the thin film guided wave SHG devices means a thickness which is capable of coinciding the effective index of a basic wavelength light of wavelength .lambda. with that of the second harmonic wave of wavelength .lambda./2, to enable phase matching of incident laser light to the second harmonic wave, and, in particular, when a SHG device for semiconductor laser is fabricated using a lithium niobate thin film formed on a lithium tantalate substrate, it is considered that a lithium niobate single crystal thin film with a thickness of over 5 .mu.m is required to coincide the effective indices.
Furthermore, in order to obtain high-output thin film guided wave SHG devices, it is necessary to increase the difference between the substrate and the thin film waveguide layer, and studies for reduced refractive indices of substrate are being conducted, for example,
3) Japanese Patent Publication 63-27681/1988 discloses a technology in which vanadium pentoxide is diffused in a lithium tantalate substrate to form a low refractive index diffusion layer with a thickness of 3-6 .mu.m, on top of which is epitaxially grown a lithium tantalate single crystal layer.
4) Japanese Patent Publication 60-34722/1985 discloses a technology in which magnesium oxide and vanadium pentoxide are simultaneously added to a lithium tantalate substrate, and a lithium tantalate single crystal layer is epitaxially grown.
However, these technologies use lithium tantalate as the thin film waveguide layer, and thus are not technologies for the formation of a lithium niobate single crystal thin film on a lithium tantalate substrate.
As described above, there have heretofore been no practical methods for the formation of a lithium niobate single crystal thin film on a lithium tantalate substrate, which is good in optical characteristics and has a sufficient thickness for the formation of waveguide optical devices such as SHG devices, optical deflectors, optical switches, and the like.
The inventors have conducted intensive studies to solve such problems. As a result, it has been found that these problems are caused by the fact that lattice constant of lithium niobate single crystal thin film differs from that of lithium tantalate substrate, resulting in a stress in the epitaxially grown crystal lattice, and found that a lithium niobate single crystal thin film which is lower in optical propagation loss than conventional thin films and has a sufficient thickness for the fabrication of optical devices can be practically produced by adjusting the lattice constant of lithium niobate single crystal thin film to match to the lattice constant of lithium tantalate substrate (lattice matching).
It has also been found that, as a means for lattice matching of lithium niobate single crystal to lithium tantalate substrate, sodium and magnesium can be contained in the lithium niobate single crystal to prevent optical damages (refractive index of crystal varied by irradiation with intense light) to lithium niobate single crystal and adjust the lattice constant of lithium niobate substrate.
Addition of sodium is described in 5) Journal of Crystal Growth 54 (1981) 572-576, in which sodium is added to lithium niobate to form a sodium-containing lithium niobate single crystal thin film with a thickness of 20 .mu.m on a Y-cut lithium niobate substrate by a liquid phase epitaxial growth technique, and 6) Journal of Crystal Growth 84 (1987) 409-412, in which sodium is added to lithium niobate to form a sodium-containing lithium niobate single crystal thin film on a Y-cut lithium tantalate substrate by a liquid phase epitaxial growth technique.
However, these literatures describe variation of lattice constant of lithium niobate single crystal by containing sodium, but these technologies relate to SAW (surface acoustic wave) devices, and do not describe a film with good optical characteristics obtained by matching the optical characteristics and lattice matching to lithium tantalate substrate. Furthermore, lithium niobate single crystal thin films described in these literatures are all for SAW devices, the film described in the former literature uses a lithium niobate substrate, and the film described in the latter literature is formed on a lithium tantalate substrate but the film and the substrate are not lattice matched, neither cannot be used in the optical materials claimed by the present invention.
Furthermore, 7) U.S. Pat. No. 4,093,781 describes a method in which, when lithium ferrite film is formed on a substrate by a liquid phase epitaxial technique, lithium is substituted with sodium to match the lattice constant to that of the substrate, thereby forming a lithium ferrite film with no stress.
However, this invention is a technology on lithium ferrite, but cannot be used for optical materials which are disclosed in the present invention.
Also, 8) Japanese Patent Publication Laid-open S52-142477/1977 describes a technology which uses a very mild crystallization starting condition to moderately grow a crystal, thereby obtaining a liquid phase epitaxial crystal with no lattice stress.
However, this technology relates to fabrication of a semiconductor thin film, but does not relate to an optical thin film suited for use as an optical waveguide which is the purpose of the present invention.
As described above, the technologies of 4) to 8) do not relate to a lithium niobate single crystal thin film with good optical characteristics.
In the fabrication of a lithium niobate single crystal thin film, it is known that lithium tantalate substrates which are often used in the prior art are inferior in crystallinity to optical lithium tantalate, since they are for use in SAW devices.
For this reason, there has been a problem that a lithium niobate single crystal thin film formed on a lithium tantalate substrate for SAW device use in difficult to obtain improved crystallinity of the lithium niobate single crystal thin film, because the crystallinity of the substrate is transferred to the thin film.
Furthermore, a flat plate-formed single crystal substrate tends to be damaged by mechanical or thermal shocks, and has been especially difficult to handle in processes which involve heating such as liquid phase epitaxial growth and heat sputtering methods.
The inventors have conducted intensive studies on lithium niobate single crystal thin films, and found that the prior art problems can be solved by using a single crystal thin film formed on a lithium niobate single crystal plate as a substrate in place of the prior art flat plate-formed single crystal, thereby accomplishing the present invention.